I was able to purchase the minie for 250 scrap. I then had to grant myself admin rights to do stuff and I removed the admin rights again and it seems like I can buy a mini. I granted the default group right 

c.grant group default vehiclevendoroptions.price.minicopter.scrap.250
 
and still not working. Im current not admin

This sounds similar to the following thread, which has some explanation on how the purchase override works technically.
https://umod.org/community/vehicle-vendor-options/44246-players-cannot-purchase

Basncially the plugin has to do two things. It needs to make the client think they have enough scrap, to allow the player to determine it can show the options to pay. It also has to hook the purchase attempt to override how much payment is taken. This is because the vanilla dialogue menu system is not inherently very moldable.

When you encounter this issue, do you see the option to pay, or do you instead just see the "can't afford" message when selecting the minicopter option? If you immediately see the "can't afford" message, that means the issue is related to the fake inventory snapshots, though I don't yet know what could cause it to not work.

So, after struggling a bit I think I found a workaround, I had hard-earned 260 scraps I got from looting barrels. I was unable to buy a mini. It was giving me the message "I can't afford"...after a few tries I remembered we had the same problem a while back testing the plugin and we got sorted once we dropped the scrap. So I divided the scrap in my inventory between 2 piles\heaps 250 and 10. I then dropped the two piles on the ground away from each other and picked them both up again. I then went back to the dealer and I was able to pay 250.

I don't think that workaround should be any different in result as just picking up the scrap in a single pile, or splitting it in your inventory, or combining it in your inventory. Any change to the contents in your inventory will trigger a network update within approximately 0.1 seconds to sync your client.

Can you confirm at which stage of dialogue that you see the issue?

1. You choose minicopter, then instead of getting an option to pay, it says you don't have enough.

2. You choose minicopter, then you see an option to pay. You choose the option to pay, then see a message saying you don't have enough.

If you see number 1, then we are facing a client synchronization issue. If option 2, then synchronization is working, but the server side logic to find the scrap is not working. Most likely, you are seeing 1, since that's been reported before, but please confirm.

option 2, I see the price of 700 and I see the price of 250 below it BUT THEN it gave me the option to say I don't have enough money

Sounds like you are describing scenario 1. Can you take a screenshot?

will do if it happens again thanks